Control systems for aircraft in the past utilized a system of cables which operated hydraulic actuators for powering the control surfaces, landing gear, and other systems controlled from the cabin. The weight and space requirements for these cable systems, and the expense of manufacturing, installing, and maintaining them in modern aircraft has inspired an effort to replace them with electronic controls which utilize electrical signals from controllers in the cabin, transmitted over wires, to actuate machinery for moving the airplane control surfaces, hence the name "fly-by-wire" system. These aviation electronic (or avionic) systems must be built with the same or better reliability and fault tolerance than the earlier cable systems had.
A goal of modern avionic systems is to integrate all of the functions for operation and control of the airplane from the cabin into a control system that permits the maximum possible reduction in the weight, space and power requirements of the avionics system, and permits a simplification in wiring between physically separated airplane control surface actuators and other control functions. Such integration would permit the consolidation of all airplane control functions in a single controller in the cabin which would make automatic control of these functions possible, without burdening the pilot or other operating personnel with routine functions, such as operating passenger cabin lights, air conditioning, seat belt signs, and so on.
This desirable integration of all control functions into one consolidated avionics system can be best achieved by the use of a common data bus to which each control function has access through an associated terminal, each of which is capable of transmitting and receiving data. Data transmitted on the data bus by one terminal associated with a particular control function can be received by the terminals associated with remaining control functions, thus eliminating the requirement for separate wiring interconnections between the main controller and each individual control function. In addition, data generated by a particular control function can be used by any other control function and is available anywhere in the airplane without the necessity of having to add separate wiring specifically to access that data.
The most desirable avionic data communication system is an autonomous terminal access data communications system that uses a current mode data bus. The items critical to the usefulness of such a data communication system are the maximum number of data terminals that may be accomodated, the ease with which each terminal is coupled to the data bus, and the reliability of the system components and the bus cable itself. A system employing a current mode data bus of high reliability having these attributes is described in U.S. Pat. No. 4,264,827, and coupling methods and protocol standards for use with this system are disclosed in U.S. Pat. Nos. 4,199,633 and 4,471,481. This system, now identified in the field as ARINC specification 629, uses a multi-transmitter, bi-directional bus instead of the older single transmitter unidirectional (one-way) bus which operated in accordance with ARINC specification 429.
A current mode data bus has the advantage that it does not require electrical connection to the conductor itself but is inductively coupled to the line replacable units with which it communicates and which communicate with it. In this way, it is unnecessary to establish metallic connection with the data bus conductor to connect the functional components to the data bus, so the inherent unreliability of plugs and sockets used in voltage mode connections to the data bus is avoided.
The data bus system described in the aforesaid patents does not require a singular data bus controller, thereby eliminating the possibility of a malfunction of the entire data bus system in the event that such a data bus controller should malfunction. Instead, each controller is autonomous, with the result that, if one controller fails, the overall data bus system continues to operate. Built-in protocols in each terminal that communicates on the bus which require them to independently listen to the bus and to transmit data when certain protocol conditions are met. For example, to receive data, each terminal passively monitors the data bus. Data received by that terminal is validated and passed into the system's addressable memory. To transmit, the terminal maintains a transmission time slot on the bus. Prior to the actual transmission, the terminal first assembles a complete data message, determines when it may transmit, and then sends the message.
The described avionic system is inherently more reliable than the older single transmitter, one-way type bus system in that it does not utilize a single controller to operate the bus and has a simpler wiring configuration. However, the increase in the required reliability of the data bus system imposes a necessity that the reliability of the associated components also increase comensurately to insure that the full benefit of the increased system reliability is realized.
In addition to high reliability, avionics data communication hardware, particularly for use in fly-by-wire systems, must meet high standards of fault tolerance. System reliability is determined in part by the design of the terminating resistor of the data bus, by the manufacturing process and materials by which it is made, and also by the method by which the terminating resistor is connected to the data bus cable itself. The terminating resistor must exceed the required reliability established by the data bus cable itself and the coupling technique used to couple the data bus cable to the line replaceable units. It has been determined that the electrical requirements of the terminating resistor could be met with a variability of 5% from the characteristic impedence Z.sub.0 of the data bus cable. In order to accomplish fault tolerance, the resistor must be designed so that failure of any single resistance element does not alter the resistor value by more than five percent.
If the data bus terminators could be shown to have a failure rate less than that of the bus cable alone, once installed and checked out, they would never need reinspection. A terminator of such a data bus can consist of a single resistor which has an extremey low failure rate, however a single point failure of the component or the connections to it causing either an open or a short can cause the data bus to fail. Fault tolerance can be built into a terminator by judicious use of redundancy in the form of parallel resistors. The reliability of the parallel array of resisters is somewhat less than that of a single resistor, but the failure rate still exceeds that of the data bus cable by several orders of magnitude based on an inspection interval equal to the service life of the airplane, and fault tolerance has now been built in.
Such a data bus cable system is advantageous because the methods of testing a data bus to insure against faults could result in decreased reliability, reduction in EMI hardness, and increased complexity and cost. Based on a 20 year service life of an airplane, which results in a projected power-on time of about 110,000 hours, the reliability of the components of a data bus system should be sufficient if they do not exceed the failure rate of the data bus cable itself, which is 10.sup.-9 failures/hour, based on experience with single segments of cable in airplanes. Such a failure rate, based on an inspection interval exceeding the life of the airplane, obviates the need for a permanent or intrusive system in the airplane to check for latent faults in the data bus system. The added complexity of such an intrusive system itself could result in additional degredation of system reliability.
The function of the terminating resistor is to absorb or sufficiently attenuate signals incident on it so that they are not reflected back into the transmission line at an amplitude where they would cause distortion of the data signal. In order to accomplish this, the resistor value must be equal or close to the characteristic impedance Z.sub.o of the transmission line. Since the exact value of the transmission line characteristic impedence is not known with certainty before the resistor is connected to the line, it would be desirable to provide a means for trimming the resistor to perfectly match its resistance value to the actual measured value of the characteristic impedence of the data bus line. Resistance trimming is a technique for increasing resistance by removing material from a film resistor using a laser or other cutting or abrading tool (such as EDM or water/abrasive jet) to adjust the resistance of the resistor. It would be useful to be able to use resistance trimming to trim parallel resistors in a parallel stack to adjust the resistance of each resistor in the stack.
An optimum manufacturing system would produce resistors that all had the same resistance value, and then provide a system for trimming all the resistors in the stack of parallel resistors to adjust each resistor an equal amount to produce the desired total resistance of the stack of parallel resistors. Such a system would ensure that the effect of the failure of any one resistor in the stack would be the same as the failure of any other resistor in the stack. This predictability facilitates the fault tolerance design of the overall data communication system.
As an aid to the manufacturing process and also for shipping and receiving inspections it would be useful to provide an instrument for confirming that the data bus with the attached terminating resistor is in fact adjusted to adequately absorb signals incident on the end of the data bus to preclude reflection of signals strong enough to interfere with the operation of the data bus. There are many test instruments for testing the integrity of a data bus to confirm that the line replacable units and the data bus controller are capable of transmitting and receiving the signals accurately, but no instrument exists for nonintrusively measuring the resistance of a resistor on the end of a data bus, that is, measuring the resistance without cutting through the insulation on the data bus wires or the insulation on the resistor itself.
The terminating resistor in an avionic data bus for an aircraft must be mechanically robust so as to resist the inevitable vibration, mechanical handling, and thermal cycling to which it will be subjected during the operation over the lifetime of the airplane. It must be capable of withstanding impacts from being dropped and so forth, pulling forces exerted by the data bus on the terminating resistor, and bending forces between the data bus and the terminating resistor. Each constituent resistor must be a separate entity to preclude failures which could propagate to more than one element or common connection. In addition, vibration, which has the potential for establishing a resonant condition in the resistance elements, must be damped or prevented, as it could shorten the life of the terminating resistor by fatigue cracking in the resistor elements. These requirements militate for packaging the terminating resistor in a manner which will protect the electrical elements in the resistor from mechanical damage which would otherwise occur from the normal expected abuse that it would experience during the lifetime of the airplane, and also ensure that the electrical characteristics remain constant within the permissible range during that same time period.
To ensure that the capacitance of each conductor of the bus cable is balanced, a metal shell is provided around the resister pack and the spacing between the conductors and the metal shell is maintained equal for both conductors by a foam sleeve. This protects the resistor pack and also ensures balanced capacitance of each conductor.